The invention relates to the field of micro-electromechanical systems (MEMS) for electrical switching, and in particular to a high frequency piezoelectrically actuated latching relay with liquid metal contacts.
Liquid metals, such as mercury, have been used in electrical switches to provide an electrical path between two conductors. An example is a mercury thermostat switch, in which a bimetal strip coil reacts to temperature and alters the angle of an elongated cavity containing mercury. The mercury in the cavity forms a single droplet due to high surface tension. Gravity moves the mercury droplet to the end of the cavity containing electrical contacts or to the other end, depending upon the angle of the cavity. In a manual liquid metal switch, a permanent magnet is used to move a mercury droplet in a cavity.
Liquid metal is also used in relays. A liquid metal droplet can be moved by a variety of techniques, including electrostatic forces, variable geometry due to thermal expansion/contraction and magneto-hydrodynamic forces.
Conventional piezoelectric relays either do not latch or use residual charges in the piezoelectric material to latch or else activate a switch that contacts a latching mechanism.
Rapid switching of high currents is used in a large variety of devices, but provides a problem for solid-contact based relays because of arcing when current flow is disrupted. The arcing causes damage to the contacts and degrades their conductivity due to pitting of the electrode surfaces.
Micro-switches have been developed that use liquid metal as the switching element and the expansion of a gas when heated to move the liquid metal and actuate the switching function. Liquid metal has some advantages over other micro-machined technologies, such as the ability to switch relatively high powers (about 100 mW) using metal-to-metal contacts without micro-welding or overheating the switch mechanism. However, the use of heated gas has several disadvantages. It requires a relatively large amount of energy to change the state of the switch, and the heat generated by switching must be dissipated effectively if the switching duty cycle is high. In addition, the actuation rate is relatively slow, the maximum rate being limited to a few hundred Hertz.
A high frequency electrical relay is disclosed that uses a conducting liquid in the switching mechanism. The relay uses an actuator, such as a piezoelectric element, to cause the switch actuator to insert into a cavity in a static switch contact structure. The cavity has sides and a pad on its end that are wettable by the conducting liquid. The cavity is filled with the conducting liquid, which may be liquid metal. Insertion of the switch actuator into the cavity causes the conducting liquid to be displaced outward and come in contact with the contact pad on the switch actuator. The volume of conducting liquid is chosen so that when the actuator returns to its rest position, the electrical contact is maintained by surface tension and by wetting of the contact pads on both the static switch contact structure and the actuator. When the switch actuator retracts away from the static switch contact structure, the available volume for conducting liquid inside the fixed switch contact structure increases and the combination of the movement of the conducting liquid into the cavity and the contact pad on the switch actuator moving away from the bulk of the conducting liquid causes the conducting liquid connection between the fixed and moving contact pads to be broken. When the switch actuator returns to its rest position, the contact remains electrically open because there is not enough conducting liquid to bridge the gap without being disturbed. The high frequency capability is provided by the additional conductors in the assembly, which act to make the switch a coaxial structure. The relay is amenable to manufacture by micro-machining techniques.